We are utilizing a non-conditional polymerase mutant of B-tropic MuLV to study the mechanism of the reverse transcriptase reaction. This mutant retains some of the catalytic activity of the wild-type enzyme but does not synthesize DNA from the natural template. The nature of this block is being investigated. In addition, mutants of tropic MuLVs are being constructed using recombinant DNA technology. It is hoped that such mutants will enable us to correlate known viral genes and gene products with specific viral functions.